I. Field of the Invention
This invention relates generally to glass fiber optical waveguides. More specifically, the invention relates to those waveguides that propagate light at least in part through the mechanism of reflection at the interface between the exterior surface of a glass core and a polymer cladding, typically silicone rubber.
II. Description of the Prior Art
The past few years have witnessed the rapid emergence of fiber optic technology from laboratories around the world into a wide variety of commercial applications. This emerging technology has given rise to many inventions, patents, books and other technical publications. A recent and quite readable overview article which contains a glossary of the more commonly used electroptical terms is authored by L. Borsuk, and entitled "What All Connector Engineers Should Know About Fiber Optics", Insulation/Circuits, August, 1978, Pages 43-49. The Oct. 25, 1978 issue of Electronic Design contains a special report assessing fiber optic technology which begins on page 53 thereof. The above identified article and special report are both incorporated by reference herein.
The final design of any fiber optic information transmission system is a complex function of many system component characteristics and engineering tradeoffs. Although many optical waveguide constructions are known, the characteristics associated with each construction vary widely and, therefore, the type of waveguide used in any given system must be matched to that system's requirements.
One optical waveguide construction which offers a number of desirable attributes has a stepped refractive index profile. Such a profile is usually obtained when a homogeneous glass core, is surrounded by a lower refractive index cladding of glass or polymer. A waveguide so constructed propagates light through the reflection of rays at the interface between the exterior surface of the glass core and the cladding.
Another optical waveguide construction uses a core with a graded refractive index profile. Typically, such profiles are formed when the refractive index decreases outwardly from the center of the core. Such a waveguide propagates light through the refraction of rays at selected regions within the core. It is to be understood that a waveguide which propagates light at least in part through the reflection of rays at the interface between the exterior surface of the core and the cladding and at least in part through the refraction of rays at selected regions within the core can be fabricated and may be useful in certain applications.
Polymer clad step index glass fiber waveguides are relatively simple to fabricate compared to other waveguide constructions. They are relatively easy to launch light into because of their large numerical apertures (typically 0.2 to 0.4) and, in theory at least, should be easier to interconnect because their relatively large core diameters (typically 125 to 400 microns) can tolerate greater axial misalignment than waveguides with smaller core diameters (typically 5 to 75 microns). In addition, polymer clad glass fiber waveguides appear to be less susceptible to the adverse effects of ionizing radiation at levels in excess of 1 Mrad than other waveguide constructions.
Large core diameter, step index waveguides allow light to propagate in many modes. Some modes have longer optical paths to traverse than others. These unequal optical path lengths result in undesirable signal dispersion over long distances. Although such dispersion ultimately limits the signal bandwidth of such waveguides, there are many applications requiring a waveguide less than one kilometer in length where some signal dispersion at selected information transmission rates can be tolerated.
A variety of polymers have been used to form the cladding layer which surrounds the core. In particular, silicone base polymers have proven useful in cladding waveguides to achieve low levels of signal attenuation (less than 10 dB per kilometer when measured at 820 nanometers). However, two well known problems associated with prior art silicone clad glass waveguides have limited their more widespread use. First, the soft, rubbery nature of silicone base polymers renders waveguides clad with such materials, difficult if not impossible to terminate without significant signal losses. Second, the inability of silicone base polymers to function as an effective moisture barrier leaves the surrounded glass waveguide core vunerable to moisture enhanced stress cracking.
Prior art efforts to overcome these problems have not been successful. For example, attempts have been made to bond the silicone to a connector ferrule or other waveguide supporting structure. The strength of such a termination is inherently limited by the strength of the glass to silicone bond and/or the shear strength of the silicone material itself. In addition, because the glass core is not securely held, it is subject to some axial, radial and rotational displacement when a remote portion of the waveguide cable is moved such as by twisting, coiling or pulling. In the situation where a waveguide is terminated in abutment to an optical source or detector, such as a light emitting diode, laser diode, photo transistor or the like, even a small amount of fiber end displacement can result in damage to these components. Fiber end motion in a waveguide connector can make the connection losses a variable function of waveguide cable placement. Moreover, in terminating a silicone clad waveguide in a connector, two additional considerations should be kept in mind. First, to achieve a low loss connection between two waveguides, it is essential that the longitudinal axes of their cores be closely aligned. Although their alignment can be accomplished in many ways, one of the most attractive is to employ a connector with an aligned bore which centers the glass core therein. As a practical matter, such centering is difficult to achieve with a silicone clad waveguide as the outside diameter and concentricity of the cladding must be precisely controlled to just fit within the connector bore while at the same time the core must be precisely centered within the cladding. Second, assuming the waveguide has been centered and secured within the connector bore, it is still necessary that the end surface of the core possess an optically smooth finish. Such a surface can be obtained by either cleaving the glass core or by polishing the end with a series of increasingly fine abrasives. Although cleaving can provide a very smooth finish, abrasive polishing is generally preferred because the polishing operation can be used to accurately establish a predetermined relationship between the core end and the connector body thereby providing better control of the lateral spacing between aligned core end surfaces. One problem encountered in the prior art when attempting to polish a core end is that the surrounding silicone cladding is too soft to provide adequate support. Therefore, the core is allowed to bend back and forth during polishing. This motion frequently causes the edge of the core to chip and break. At best, the core end becomes polished with a slightly convex rather than flat surface which increases attenuation.
Attempts have been made to avoid some of these problems by using silicone base polymers with a variety of additives to make the cladding stiffer and therefore easier to work with. These attempts have all adversely effected the overall attenuation characteristics of the waveguide.
It would appear that using a hard and rigid low refractive index polymer as the cladding would be advantageous. In practice, it has been found that if such a material is used for the cladding, then unacceptable microbending losses are induced in the waveguide. Soft polymeric and specifically silicone rubbers when used as the waveguide cladding are advantageous in that they help to cushion the waveguide from induced microbending losses.
Another prior art approach to terminating silicone clad waveguides has been to remove an end portion of the cladding and form an adhesive bond between the exterior cylindrical surface of the core and the bore of a connector or other waveguide supporting structure. If an adhesive is used which does not have a lower refractive index than the core, then a high loss termination is produced.
The minimum low refractive index cladding thickness is at least 3 and preferably at least 12 times the wavelength of the light to be propagated. If this minimum thickness criteria is not met, then undesirable light loss from the core will also result. Therefore, even if an adhesive with a sufficiently low refractive index is used, it is still important that the core be spaced sufficiently apart from the waveguide support structure to avoid light losses.
Prior art efforts to terminate silicone clad waveguides have also included dipping an end of a glass core with no cladding into a solution of a dissolved, heat fusible polymer having a refractive index lower than the refractive index of the exterior surface of the glass core. The core is slowly withdrawn from the solution and warmed with hot air thereby causing the solvent to evaporate and the polymer to fuse into a thin layer. In theory, this procedure can be repeated as many times as necessary to build up as thick a polymer layer as is desired. In practice, the polymer residue left on the surface of the core after the cladding has been stripped off is difficult to remove entirely and is difficult to bond to. As a result of these difficulties, the polymer layer formed by dip coating is frequently not entirely bonded to the glass and not uniformly distributed around the core. Not only is this procedure time consuming and craft sensitive, but also, it does not provide a practical solution to the problem of centering and bonding a small diameter core within a bore of an inexpensive waveguide connector. This technique is more fully described in the article by R. L. Warkentine, entitled "To Terminate Plastic Clad Silica Fibers, First Strip and Reclad the Ends", Electronic Design, Oct. 25, 1978, Pages 118 and 119.